Optical fibers have been widely used in high speed communications links. Optical links using fiber optics have many advantages compared to electrical links: large bandwidth, high noise immunity, reduced power dissipation and minimal crosstalk. Fiber optic communications links can have data transmission capacity in the terabit per second range. In communications systems where optical fibers are used to transport optical communications signals, various optoelectronic devices are used to control, modify and process the optical signals.
Wavelength division multiplexing (WDM) is an essential technique for the full utilization of the large bandwidth available for use in optical fibers in optical communications systems. Various technologies and devices have been developed for the multiplexing and demultiplexing of many channels of light in a WDM system. Devices such as thin film interference filters, fused fiber couplers, Mach-Zender interferometers, fiber Bragg gratings, bulk diffraction gratings, optical circulators, arrayed waveguide gratings (AWG) and other devices have been used to fabricate multiplexers and/or demultiplexers.
However, there are drawbacks associated with these WDM technologies. Many of these systems, such as the MZI, Bragg gratings and AWG are incompatible with multi-mode optical fibers. Thin film filters and bulk diffraction grating can be used with multi-mode fibers, but this requires use of free-space optical systems, which require more parts and precise alignment, resulting in costly solutions.
Light emitted from multi-mode fibers (MMF) differs from that in single mode fibers (SMF) in two key ways. First, the spot size of the light from a multi-mode fiber is larger than a single mode fiber. MMF typically has a 50 or 62.5 micron core, whereas SMF typically has an 8–10 micron core. While these are typical dimensions, it is commonly understood that other dimensions can be employed for both types of fiber. The second main difference is in the modal distribution of the light. MMF has light that exits the fiber in a large number of different spatial states and two fundamental polarization states, whereas SMF has a single spatial state and two fundamental polarization states. When the large number of spatial states present in MMF are coupled into a waveguide based WDM filter, often light in the higher order modes is lost, due to the preference of WDM filters to be designed for single mode input. Thus, waveguide based WDM filters are not easily used in systems utilizing MMF. It would be ideal to have a WDM filter that worked for MMF, and still have the benefits of waveguide based operation, such as planar construction and the ability to incorporate waveguide photodetectors. Another ideal property would be a device capable of operation in both MMF and SMF systems.
Waveguide grating couplers are well known devices for coupling light from a waveguide mode to an external mode, often a fiber or a free-space beam. One property of these grating couplers is a coupling passband between the external mode (fiber or free space), and the waveguide modes. Another property of waveguide grating couplers is the polarization dependence of the coupling. There are designs well known by those skilled in the art to operate only on a single polarization state. Examples of waveguide grating couplers are discussed in U.S. patent application Ser. No. 10/776,475 entitled “Optical Waveguide Grating Coupler” filed on Feb. 11, 2004 and U.S. patent application Ser. No. 10/776,146 entitled “Optical Waveguide Grating Coupler with Scattering Elements of Varying Configurations,” filed on Feb. 11, 2004, which are incorporated herein by reference.
In addition, another class of waveguide grating coupler treats multiple polarizations simultaneously, though in different manners. Examples of polarization splitting grating couplers are discussed in U.S. Pat. No. 6,788,847, entitled “Photonic Input/Output Port” and U.S. patent application Ser. No. 10/734,374 entitled “Polarization Splitting Grating Coupler’ filed on Dec. 12, 2003, all of which are incorporated herein by reference.
A final category of waveguide grating coupler is one that is designed to perform identically on each polarization state. A key property of waveguide grating couplers is that they can be lithographically defined in desired orientations, shapes and sizes. This allows construction of a number of different grating couplers in a region of a waveguide.
Integrated optoelectronic devices made of silicon are highly desirable since they can be fabricated in the same foundries used to make VLSI integrated circuits. Optoelectronic devices integrated with their associated electronic circuits can eliminate the need for more expensive hybrid optoelectronic circuits. Optoelectronic devices built using a standard CMOS process have many advantages, such as: high yields, low fabrication costs and continuous process improvements.
Recent developments in the fabrication of low loss, high speed grating couplers on silicon or SOI substrates have provided the possibility of new applications for such devices.